A traditional shoe has an upper which receives a foot of a wearer, and a sole having a midsole and an outer sole, or outsole, connected to the upper. The upper has a front portion for receiving the toes and front portion of the foot of the wearer, and a rear portion for receiving the rear portion of the foot of the wearer including the heel of the wearer. As the wearer walks or runs, the load of the wearer's body is exerted primarily in two separate locations of each of the wearer's feet. In particular, as the wearer walks or runs, the wearer advances one leg forward along with his/her first foot, and upon contact of the outer sole of the shoe with the ground, the heel of the first foot will exert a downward force or load, with a center of such force being exerted generally from the center of the wearer's heel of the first foot. The center of this force exerted by the rear portion of the first foot can be considered the rear center of loading.
As the leg moves from this forward position to a position below the torso and rearward of the torso, this force or load exerted from the heel of the first foot will reduce and transfer to the front portion of the first foot. The load will then transfer to the front center of loading. The front portion of the first foot has a front center of loading. The front center of loading extends generally along a line from the center of the “ball” of the foot toward the exterior of the foot in a path which is generally parallel to the toes.
Using shoes for walking, running, and other activities for an extended period of time, distance, or both can cause fatigue to the wearer, including fatigue to at least the muscles, tendons, ligaments, and cartilage of at least the feet, legs, and torso. This fatigue can be caused by several factors, such as the impact forces resulting from the change in the rate of change of loading or “bottoming out” of conventional shoe materials.
Recent research in running mechanics (see “Impact Forces in Running” by Dr. Benno M. Nigg, 1997) explains that neither the magnitude nor duration of impact forces experienced during running is the primary cause of running fatigue or injuries. The injurious factor in running is a physiological coping mechanism known as “muscle tuning.” Muscle tuning is the body's response to the sharp rise in impact force the body experiences during the initial phase of the stride. When impact forces rapidly rise, as during a stride in current running shoes, the body's large muscle groups momentarily tense to prevent the body's soft tissues, large muscle groups and internal organs, from shaking or vibrating in response to the onset of a rapidly-rising impact force. This muscle tuning effect varies according to each runner's physiology and performance profile.
Muscle tuning is the source of localized neuromuscular fatigue. Factors affecting muscle tuning include at least stride length, strength, cardiovascular fitness level, body mass index, weight, fatigue level and tissue hydration level. The muscle tuning effect is often quite pronounced and leads to cumulative fatigue and diminished endurance. These same stride forces have also been implicated as the dominant factor in stress fractures. Therefore, a shoe that allows the wearer to stride with minimal muscle tuning and neuromuscular fatigue is preferred. However, prior shoes do not manage impact forces in such a way as to minimize muscle tuning. Some remedial efforts have been made in an attempt to reduce fatigue.
U.S. Pat. No. 4,881,329, issued Nov. 21, 1989 to Crowley, is directed to an athletic shoe with an energy storing spring. Crowley discloses a spring positioned within the heel portion of the midsole of the shoe. Using midsole material above and below the spring diminishes the effectiveness of the spring. In addition, limiting the spring element's location to being laterally within the midsole can cause stability problems.
U.S. Pat. No. 6,282,814 B1, issued Sep. 4, 2001 to Krafsur et al., is directed to a spring cushioned shoe. Krafsur et al. discloses a sole assembly having a first spring disposed within a vacuity in the heel portion of the assembly, and a second spring disposed within a vacuity in the ball portion of the assembly. The vacuities are within the midsole of the shoe. The springs are “wave” springs and are made of a metal material, which can cause the shoe to become heavy and inflexible, thereby reducing the efficiency of the shoe.
U.S. Pat. No. 4,910,884, issued Mar. 27, 1990 to Lindh et al., is directed to a shoe sole incorporating a spring apparatus. Lindh et al. discloses a shoe sole with a cavity in its upper side. Two elliptical springs are situated entirely in the cavity, and fit snuggly but freely in the cavity. A flexible bridge piece fits over the springs. The bridge is a flat spring of uniform thickness, having a planform conforming to the planform of the cavity such that it fits freely but closely in the cavity in the sole. This arrangement suffers from at least the deficiencies of Crowley, and additionally may cause unwanted strains on the user's feet, difficulty in manufacture, and a lack of a cohesive (one piece) feel to this shoe in view of the springs not being integral with the sole.
The present invention is provided to solve these and other problems.